DESCRIPTION: This proposal outlines studies to dissect the mechanisms regulating alternative splicing in the Drosophila homeotic gene, Ultrabithorax (Ubx). Alternative splicing of the primary transcript of Ubx results in 6 mRNAs, each encoding a distinct protein isoform. All isoforms vary in sequence in an internal domain and have identical N and C terminal domains. The internal domain is specified by three combinatorial coding elements. The B element is an extension of exon E5' bracketed between two alternative donor splice sites designated a and b. It specifies 9 amino acids. Element MI is a microexon encoding 17 amino acids, as is element MII. Alternate forms have various combinations of MI and MII elements spliced to E5', from either the a or b donor sites, with the b site incorporating the 9 amino acids of element B into the protein. When splicing occurs, potential donor splice sites are regenerated at the E5'/mI, E5'/mII and mI/mII boundaries. The alternate forms are developmentally regulated, with central nervous system, peripheral nervous system, epidermis and mesoderm expressing distinctive patterns of alternate forms. Because these forms are expressed in different combinations and quantities, rather than in discrete, "one form only" patterns, it appears that regulation of alternative splicing of Ubx allow a fine-tuning of expression and function rather than dictating the direction of a developmental switch. At issue in this project, is how this form of alternative splicing regulation is accomplished.